Add ability to display state labels

[pdil]

There should be an option in plot_usmap to display state abbreviation labels (AK, AL, AR, etc.) in the center of each state polygon.

This may look something like this:

plot_usmap(data, ..., labels = true)

labels would be the new parameter and accept true or false values (default being false) indicating whether or not to display labels.

Another new option could be the ability to set label color.

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